Surface drainage consists of components that are able to capture runoff water from areas such as driveways, hardscapes, roof tops, and landscaped areas. Surface drains allow drainage water to be piped to proper areas in order to help minimize its effect in the landscape. Surface drains should be used in areas where water can be easily collected and diverted into catch basins. By collecting surface water quickly, it can be redirected to prevent saturating the soil in unwanted areas such as building foundations.

In order to size drainage catch basins and drainage pipe correctly, it is necessary to know how much runoff water is on your site. There are several factors that will affect the size of materials used to collect unwanted drainage water. The staff at W.P. Law Inc. has developed a Drainage Calculator that will allow you to enter information about your site to assist you in properly sizing the necessary drainage components.

Note: The yellow boxes are drop down menus. The gray boxes allow for manual entries and the blue boxes are calculated values.Step #1
The first step involves calculating the area of where you will be collecting the runoff water. Most sites will be divided up into sections of where the water collects. Use the Drainage Calculator to calculate the area of the various sections. To determine how much water is running off of building rooftops, evaluate the various gables and slopes to calculate each area separately. You can enter in the dimensions of the shape you are calculating for or if you already know the square footage, type it into the lower box.

Step #2In the second step of the drainage calculator, enter in the soil or cover type for the area to be collected. This box is important because surfaces vary in permeability. Surfaces such as asphalt or concrete do not absorb rainfall while grassed areas on sandy soils will soak in a certain amount of rainfall. Select the correct soil or cover type from the drop down menu or if you know the runoff coefficient, you can enter it into the second box.

Step #3This step allows you to enter in information about the location of your site. By entering this information, the calculator determines the maximum inches per hour of rainfall for a 100 year rainfall event. If you already know this information, you can enter it into the second box, if not select the proper area of South Carolina for your site from the first drop down box.

Note: Once you complete Step #3, the Drainage Calculator will automatically calculate the size of the drainage catch basin you will need and display this information in box #5. Now that the drainage catch basin has been sized, proceed to Step #4 to determine the necessary pipe size needed to carry the amount of water to be collected.

Step #4In order to calculate the proper pipe size, you will need to determine the percentage of slope from the drainage catch basin to the outlet of the pipe. If you know your percentage of slope, enter it into the first box. If you need to calculate the percent slope, enter the total length of pipe run (feet) and the total drop (inches) into the second and third box. Once you enter those two numbers, a calculated percent slope will appear on the right side. Enter this number into the first box.

The calculator will then calculate the size pipe to be used. Because various types of drainage pipe have differing flow characteristics, we have calculated pipe sizes for PVC sewer and drain pipe as well as single and double wall corrugated pipe.

Step #5This step calculates for round and square drainage catch basins. If this is the type of drainage catch basin you are planning to use, it will already be calculated for you. If you are planning on using a channel or strip drain, enter the length of drain needed in the lower box to determine if it will handle to amount of flow or runoff necessary.

Subsurface drainage differs from surface drainage since it collects water that has already permeated into the soil. To help resolve drainage issues, surface water should be collected with a drainage catch basin wherever possible. Due to varying site conditions however, it is not always feasible or practical to capture surface water. In these situations, subsurface drainage can be installed to help alleviate excessive moisture in the soil. Click below to learn more about the types of products and applications used for subsurface drainage and how to install them properly.